1. Field of the Invention
The present invention relates to backlight units, and more particularly, to a backlight unit in a display device having a Divided Display Area Method (DDAM) of a Field Sequential (FS) driving method applied thereto, in which a display area is divided into sections in driving the display area, and to a liquid crystal display device therewith.
2. Discussion of the Related Art
As one of display devices used generally, CRTs (Cathode Ray Tubes) are mostly used, starting from TV receivers to monitors in measuring instruments, information terminals, and the like. The CRT, however, fails to deal with the demand for small sized light weight products due to the weight and bulk of the CRT itself.
Accordingly, under the trend that various electronic products become smaller and lighter, the CRT has certain limitations in view of its weight and bulk. As displays that are expected to replace the CRTs, there are Liquid Crystal Display (LCD) which utilizes an electro-optical effect, Plasma Display Panel (PDP) which utilizes gaseous discharge, Electro Luminescence Display (ELD) which utilizes an electro-luminescence effect, and the like. Among the various display devices, researches on the LCD are active.
For replacing the CRT, the LCD, which is advantageous in view of its small size, light weight, and low power consumption, has been developed actively to be used as a flat display device with satisfaction, not only as a monitor in a laptop computer, but also as a monitor of a desktop computer and a large sized information display device. As a result, the demand for the LCD is increasing.
A driving principle of the liquid crystal display device lies on optical anisotropy and polarity of a liquid crystal. Thin and elongated structures of the liquid crystal molecules are liable to be arranged in one direction, enabling to control a direction of molecular arrangement by applying an electric field to the liquid crystals, artificially. Therefore, if the direction of the molecular arrangement of the liquid crystals is changed, the molecular arrangement of the liquid crystals is changed to make a light to refract in the direction of the molecular arrangement of the liquid crystals, thereby enabling display of picture information.
Presently, Active Matrix LCD (AMLCD) in which thin film transistors, switching devices, and pixel electrodes connected thereto are arranged in a matrix is the most interested owing to its excellent resolution and moving picture reproduction capability.
A related art liquid crystal display device (LCD) which reproduces a picture by such a driving principle will be described referring to FIG. 1, which illustrates a section of a related art LCD, schematically.
Referring to FIG. 1, the related art LCD is provided with a liquid crystal display panel having first and second transparent glass substrates 1 and 10 bonded together with a gap therebetween, a liquid crystal layer 15 filled between the first and second glass substrates 1 and 10, and a backlight unit 16 on a back surface of the first glass substrate 1 for supplying a light to the liquid crystal display panel.
Formed on the first glass substrate 1 (a TFT array substrate), there are a plurality of gate lines arranged in one direction at regular intervals, a plurality of data lines arranged in a direction perpendicular to the gate lines at regular intervals, a matrix of pixel electrodes 2 in respective pixel regions each defined at a cross region of the corresponding gate line and the corresponding data line, and a plurality of thin film transistors (TFTs) 3 each arranged to be switchable in response to a signal on the corresponding gate line for transferring a signal on the corresponding data line to the corresponding pixel electrode.
Formed on the second glass substrate 10 (a color filter substrate), there are a black matrix layer 11 for shielding a light incident on regions excluding the pixel regions, a color filter layer 12 of R (Red), G (Green), and B (Blue) cells for transmitting a portion of a light of a particular waveband and absorbing the rest portions of the light, and a common electrode 14 for reproducing the picture. A reference symbol 13 denotes an overcoat layer.
The first and second substrates 1 and 10 are bonded with sealant having a liquid crystal injection hole formed therein and a gap formed by spacers, and have liquid crystals provided therebetween.
For convenience sake, FIG. 1 illustrates only one pixel region of the first and second glass substrates 1 and 10. The LCD requires a separate light source, i.e., a backlight unit 16 for adjusting an external light incident on the liquid crystal display panel, to display a picture.
A related art backlight unit will be described referring to FIG. 2, which illustrates a diagram for describing the related art backlight unit usable in the LCD of FIG. 1.
Referring to FIG. 2, the related art backlight unit is provided with a fluorescent lamp 21, a light guiding plate 22, a diffusion material 23, a reflective plate 24, a diffusion plate 25, and a prism sheet 26.
Upon application of a voltage to the fluorescent lamp 21, residual electrons present in the fluorescent lamp 21 move to an anode, during which the residual electrons collide with, and excite argon Ar to proliferate positive ions that hit onto cathode, to emit secondary electrons.
When the secondary electrons flow through the tube to start discharge, a flow of the electrons caused by the discharge collide with mercury vapor, and ionize the mercury, to emit a UV ray and a visible light. The UV ray excites a fluorescent material coated on an inside wall of the lamp, such that the fluorescent material emits a visible light.
The light guiding plate 22 is a wave-guide for receiving the light from the fluorescent lamp 21 and emitting a surface light toward an upper side, and is formed of PMMA (Poly Methyl Meth Acrylate) which has a good light transmittivity.
Parameters related to the light reception efficiency of the light guiding plate 22 are a thickness of the light guiding plate 22, a diameter of the lamp 21, a distance between the light guiding plate 22 and the lamp 21, a shape of the lamp reflective plate. In general, the light reception efficiency increases by placing the fluorescent lamp 21 aside in a thickness direction from a centerline of the light guiding plate 22.
For the light guiding plate 22 of the LCD backlight unit, there are printed type light guiding plates, V-cut type light guiding plates, and spread light guiding plates.
The diffusion material 23 is composed of SiO2 grains, PMMA, solvent, and the like. The SiO2 grains are porous, and used for scattering the light. The PMMA is used to attaching the SiO2 grains to an underside surface of the light guiding plate 22.
The diffusion material 23 is coated on the underside surface of the light guiding plate 22 in dot forms, with an area of the dot increasing in an increment for obtaining a uniform surface light at an upper side of the light guiding plate 22. That is, an area ratio of the dot per unit area is smaller on a side close to the fluorescent lamp 21, and the area ratio of the dot per unit area is greater on a side far from the fluorescent lamp 21.
Here, the dot may have a variety of forms; however, the same luminance effect can be obtained at the upper side of the light guiding plate 22 regardless of the form of the dots as long as the area ratio of the dots per unit area is the same.
The reflective plate 24 on a back of the light guiding plate 22 directs the light from the fluorescent lamp 21 to an inside of the light guiding plate 22.
The diffusion plate 25 on the light guiding plate 22 having a dot pattern coated thereon provides a uniform luminance regardless of a viewing angle, is formed of PET or PC (Poly Carbonate) resin, and has a grain coated layer thereon for diffusing the light.
The prism sheet 26 is provided for enhancing a frontal luminance of the light transmitted to the upper side of the diffusion plate 25 and reflected. The prism sheet 26 transmits only a light incident thereon at a particular angle, and reflects other lights incident thereon at angles other than the particular angle to return such lights to a lower side of the prism sheet 26. The returned light then passes through the diffusion plates 22 and 25 and is reflected at the reflective plate 24 attached to the underside of the light guiding plate 22.
The backlight unit is fixed to a mold frame, and a display unit on top of the backlight unit is protected by a top sash. That is, the top sash and the mold frame are held together with the backlight unit and the display unit held therebetween.
However, the related art LCD has the following problems.
First, the low light transmittivity of the color filter layer of below 33%, with a great loss of the light, requires making the backlight brighter, which in turn leads to a greater power consumption.
Second, since the color filter layer is very expensive in comparison to other materials, the production cost of the LCD increases.
What is suggested for solving these problems of the LCD is the Field Sequential (FS) type LCD which can provide full colors without the use of the color filter layer of FIG. 1.
Although the related art backlight continuously supplies a white light to the liquid crystal display panel in a turned-on state, the FS type LCD selectively turns on R, G, B light sources of R, G, B backlight units for a frame at regular time intervals in sequence, for displaying a color picture.
Although the FS type was suggested around 1960, the practical application of the FS type has been difficult to achieve because technologies on a fast response liquid crystal mode, and a light source that meets the fast response are required to follow thereafter.
However, owing to the recent remarkable development of LCD technologies, the FS type LCD is suggested in which Ferroelectric Liquid crystal (FLC), Optical Compensated Bend (OCB), or TN (Twisted Nematic) liquid crystal mode having the fast response, and R, G, B backlights which can be turned on/off at a high speed are employed.
Particularly, as the liquid crystal mode for the FS type LCD, the OCB mode is used mostly. The OCB cell has upper and lower substrates having facing surfaces thereof rubbed in the same direction, and a certain voltage applied thereto, to make the liquid crystal molecules to move at a fast speed upon the application of a voltage thereto, which makes a time period required for rearrangement of the liquid crystal molecules, i.e., a response time period, very short, around within 5 msec. According to this configuration, since the OCB mode liquid crystal cell leaves no residual image on a screen owing to the fast response characteristic, the OCB mode liquid crystal cell is suitable to the FS type LCD.
FIG. 3 illustrates a section of a related art FS type LCD, schematically.
Referring to FIG. 3, the related art FS type LCD is provided with an upper substrate 30, a lower substrate 35 which is an array substrate, a liquid crystal layer 38 filled between the upper and lower substrates 30 and 35, and a R, G, B three color backlight unit 39 for supplying a light to a liquid crystal display panel of the upper and lower substrates 30 and 35 and the liquid crystal layer 38.
On surfaces of the upper and lower substrates 30 and 35 facing the liquid crystal layer 38, there are a common electrode 32 and a pixel electrode 36 for serving as electrodes that apply a voltage to the liquid crystal layer 38.
Between the upper substrate 30 and the common electrode 32, there is a black matrix 31 for shielding a light incident on regions excluding the pixel electrodes 36 on the lower substrate 35.
On the lower substrate 35, there is a thin film transistor T 37 (a switching device electrically connected to the pixel electrode 36) formed at a position opposite to the black matrix 31 on the upper substrate 30.
The thin film transistor T 37 is provided with a gate electrode, and source and drain electrodes, which are not shown. An unexplained reference numeral 40 denotes an overcoat layer.
For convenience sake, only one pixel region of the upper and lower substrates 30 and 35 is shown in FIG. 3.
The features of the FS type LCD that are most distinctive from the related art LCD are that the color filter is not needed and the R, G, B light sources of the backlight unit are separately turned on/off.
For convenience sake, the backlight unit with the R, G, B light sources will be called as a R, G, B backlight unit.
The R, G, B backlight unit 39 is driven by one inverter (not shown), such that each of the R, G, B light sources are lit 60 times per a second, total of 180 times, to cause a residual effect and to mix the R, G, B colors for expressing a color.
The 180 times of lighting on and off of the R, G, B light sources in the R, G, B backlight unit 39 are not noticeable to a viewer such that the overall effect is as if the light sources are kept lit. For example, successive lighting of the R light source and the B light source makes a person to view a violet color due to the residual effect.
That is, the FS type LCD, without the color filter layer, is suitable for a large sized LCD, because the FS type LCD overcomes the problem(s) of the related art LCD of poor overall luminance caused by the low light transmittivity of the color filter. The FS type LCD expresses full colors by means of the three color backlight unit, thereby enabling to provide a liquid crystal display panel having a high luminance, a high definition, and low cost owing to the omission of the expensive color filter layer. As described, even if the related art LCD is behind the CRT in terms of price and definition, the FS type can solve such problems.
As described, since most of the LCDs are light receptive devices in which an external light incident thereon is adjusted to display a picture, a separate light source for directing a light to the liquid crystal display panel, i.e., the backlight unit, is essential.
In general, among the backlight units of the LCD, there are an edge type and a direct type depending on the type of arrangement of the lamps.
The direct type backlight unit has lamps arranged on a flat surface, but has a limitation in fabrication of thin LCD because it is required to maintain a gap between the lamps and the liquid crystal panel for preventing a form of the lamp from showing on the liquid crystal panel, and to arrange light spreading means for uniform distribution of light on the whole device.
Moreover, the greater the size of the panel, the larger the area of a light emissive surface of the backlight unit. In the case of a large sized direct type backlight unit, since the light emissive surface can not be flat if an adequate thickness of the light spreading means can not be secured, it is required to secure an adequate thickness of the light spreading means so that the light emissive surface can be flat.
In the meantime, the edge type backlight unit having lamps arranged at an outer side for spreading a light to the entire surface of the device by using a light guiding plate, has a problem of poor luminance because the light is required to pass the light guiding plate. Moreover, for uniform distribution of the light, high optical design and fabrication technologies are required.
Since both the direct type backlight unit and the edge type backlight unit have disadvantages of their own, the direct type backlight unit is used in an LCD that requires effective luminance rather than a thin screen, and the edge type backlight unit is used in an LCD that requires thinness rather than luminance, such as a notebook PC or a monitor PC.
FIGS. 4A and 4B illustrate sections of related art FS type LCD backlight units respectively, wherein FIG. 4A illustrates an edge type backlight unit and FIG. 4B illustrates a direct type backlight unit.
The edge type R, G, B backlight unit 40 in FIG. 4A is provided with R, G, B light sources arranged in a line at one side or opposite sides of the liquid crystal display panel 41, and a light guiding plate and a reflective plate, both of which are not shown, for receiving and spreading a light from the light sources. A Cold Cathode Fluorescent Lamp (CCFL) is mostly used as the light source, and the edge type R, G, B backlight unit 40 is suitable for application to a portable computer because the computer is light and thin, and has a low power consumption.
The direct type R, G, B backlight unit 45 in FIG. 4B is provided with R, G, B light sources 46 under the diffusion plate 47, for directing a light to the entire surface of the liquid crystal display panel, in which a plurality of units of the R, G, B light sources 46 are arranged in a line horizontally.
The direct type backlight unit, mostly used in a display device that regards the luminance important, has a high power consumption because the direct type itself is thick and spreads light effectively for maintaining uniformity of the luminance.
FIG. 5A illustrates a portion of an array substrate provided for describing a driving method of a related art FS type LCD.
Referring to FIG. 5A, formed on the lower substrate (generally an array substrate) of the LCD, there are gate lines 50 running in a width direction, data lines 51 running in a direction perpendicular to the gate lines 50, thin film transistors T each at the cross of the corresponding gate line 50 and data line 51, and pixel electrodes 52 electrically connected to the thin film transistors T.
The related art LCD is driven by applying a picture signal to the data line 51, and applying electric pulses to the gate line 50 in a scanning fashion.
In driving the LCD by applying a gate pulse voltage to the gate lines 50 selectively, a line sequential driving method is used for improving a picture quality, in which the voltage is applied line by line while moving to an adjacent line in succession by using a gate scanning input device until the gate pulse voltage is applied to all the gate lines 50 when one frame is completed.
That is, upon application of the gate pulse voltage to an (n)th gate line, all the thin film transistors T connected to the gate line having the gate pulse voltage applied thereto are turned on at the same time, and a picture signal on the data line is accumulated on the liquid crystal cell and a storage capacitor through the corresponding thin film transistors T that are turned on.
Eventually, the liquid crystal molecules in the liquid crystal cell are rearranged according to the data picture signal accumulated on the liquid crystal cell, and the light from the backlight unit passes to reproduce a desired picture.
FIG. 5B illustrates a time chart of the related art FS type LCD driving method.
In general, in driving the FS type LCD, after scanning all the thin film transistors for each of the R, G, B light sources, the R, G, B light sources are lit respectively when the liquid crystals are rearranged following the application of the next voltage. That is, for the entire driving area, each of the light sources in the backlight unit is lit once for one frame.
It is required that each of the light sources completes this driving process within 1 cycle (f/3, f: frame frequency). That is, as shown in FIG. 5B, in view of one light source, one cycle is:F/3 (55)=tTFT(56)+tLC(57)+tBL(58)                Where, F: one frame time,                    tTFT: a scanning time of the entire thin film transistor,            tLC: a response time period of an assigned liquid crystal, and            tBL: a backlight flash time period.                        
In this instance, if tBL(58) is fixed and if tTFT(56) increases according to a design condition of the LCD, a required size of tLC(57) is reduced because an interval between frames is fixed. If tLC is reduced which leads to prolonging the actual response time period of the assigned liquid crystal beyond the allotted response time period of the assigned liquid crystal, the backlight is lit before the assigned liquid crystal is rearranged properly, which causes a problem of non-uniform distribution of a picture color.
FIG. 6 illustrates a sequence diagram of a frame unit color picture display of the related art FS type LCD and depicts application of R, G, B lights onto an LCD panel 61 for difficult time durations. In displaying a color picture on the related art FS type LCD, one frame time period (F) is set to 1/60 seconds, and each of the R, G, B three color light sources of the R, G, B backlight unit is turned on/off in sequence for 1/180 seconds (5.5 msec) within the 1/60 seconds. In this instance, the actual turn-on time period of each of the R, G, B light sources in one frame is shorter than 1/180 seconds, for preventing R, G, B colors from interfering with each other if the R, G, B light sources reproduce pictures in a continuously turned-on state.
Referring to FIG. 6, the related art FS type LCD displays color pictures by turning on/off the R, G, B light sources 60a, 60b, and 60c for 1/180 seconds respectively in succession for three sub-frames s1, s2, and s3 in one frame F. The frame F is a basic unit of a picture, for R, G, B respectively.
The related art FS driving method has in general a slow liquid crystal response speed such that it is difficult to drive all the gate lines within one frame. In order to solve this problem, the DDAM (Divided Display Area Method) is used in which a display area is divided into many sections.
A backlight unit in the related art LCD driven by the DDAM will now be described referring to FIG. 7.
Referring to FIG. 7, the related art LCD driven by the DDAM is provided with two LED light sources 72 at opposite sides of a light guiding plate 71 on an underside surface of a liquid crystal display panel (not shown) for illuminating the liquid crystal display panel, thereby enabling to display a picture even in a dark place.
Each LED light source 72 has a plurality of R, G, B LED lamps 73 arranged one dimensionally on a PCB (printed circuit board) or FPC (flexible printed circuit) substrate. The light guiding plate 71 is divided into 4 regions 71a-71d for driving a liquid crystal screen in succession. The liquid crystal screen is divided into first to fourth areas corresponding respectively to the first to fourth regions 71a, 71b, 71c, and 71d of the plate 71. The light guiding plate 71 is not physically separated into the four areas, but is integrally divided by operation. That is, no gaps exist between the divided regions 71a-71d because they are all integral part of one light guiding plate 71.
The R, G, B LED lamps 73 are lit by applying a voltage to each of the R, G, B LED lamps 73 for illuminating the divided regions separately, to spread the R, G, B lights within the light guiding plate 71 and to illuminate the back surface of the liquid crystal display panel in succession.
Thus, one of the R, G, B LED lamps 73 in the LED light source 72 relevant to a divided region of the light guiding plate 71 to be driven is lit in succession, so that the liquid crystal display panel reproduces a picture in a field sequence.
However, at the time of the sequential driving by sequential lighting of one of the R, G, B LED lamps 73 relevant to a divided region of the light guiding plate 71 to be driven in succession (driven by DDAM), light leakage to the adjacent regions of the light guiding plate and the liquid crystal display panel other than the driving region drops the display performance and deteriorates the quality of the image being displayed.